Grantee Research Project Results
2007 Progress Report: Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons
EPA Grant Number: R831084Title: Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons
Investigators: Kamens, Richard M.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Chung, Serena
Project Period: July 28, 2003 through July 27, 2006 (Extended to July 27, 2008)
Project Period Covered by this Report: July 28, 2006 through July 27,2007
Project Amount: $400,000
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter
Objective:
This project focuses on the elucidation of the fundamental chemistry that brings about the secondary organic aerosol formation (SOA) from aromatic reactions in the atmosphere. The overall goal is to develop a “new generation” aromatic chemical mechanism that can integrate newly discovered particle phase heterogeneous processes with the known gas phase chemistry, as a unified, multi-phase, chemical reaction mechanism, which will ultimately permit the prediction of SOA formation in the aromatic system.
Figure 1. Comparison of model simulated and measured concentrations of toluene, NOx, ozone and aerosol for two toluene/NOx experiments. The solid lines (—) represent model simulations. The pluses (+) are measured NO; asterisks (*) are measured NO2; triangles (Δ) are measured O3; diamonds (◊) are measured toluene; solid diamonds () are measured front filter mass; solid circles (•) are filter masses corrected by backup filter subtraction; and squares () are calculated SMPS particle mass concentrations by assuming density of 1.4g cm-3.
Progress Summary:
Over the course of this project we developed a chemical mechanism for toluene which reasonably predicts the gas phase chemistry and also predicts the SOA formation from toluene oxidation (Figure 1). Two papers have appeared in the peer reviewed literature on this mechanism. Our model includes aerosol phase chemistry that includes nucleation, gas-particle partitioning and particle phase reactions as well as the gas-phase chemistry of toluene and its degradation products are represented. A series of experiments that cover a wide range of temperature, solar condition and initial reactant concentrations, were carried out in the UNC 270 m3 dual outdoor aerosol smog chambers. Data obtained from these experiments were used to develop and test the mechanism. The model adequately simulates the decay of toluene, the NO to NO2 conversion, and ozone formation. Although it provides a reasonable prediction of SOA production under different conditions that range from 15 to 300 μg m-3, the model tends to underestimate the initial particle burst for almost all high toluene concentration experiments.
The main contribution that this study makes to science is that describes an atmospheric aromatic mechanism that: 1. simultaneously considers gas phase reactions, trace gas phase-particle phase partitioning, and subsequent particle phase reactions; 2. it proposes a simple chemical mechanism for particle phase nucleation; 3. permits one to distinguish between gas-particle partitioning of SOA, and heterogeneous SOA formation; 4. demonstrates the relative importance of organic nitrate formation at high and low toluene concentrations; and 5. successfully simulates gas phase toluene oxidation in smog chamber systems. To date, this has not been accomplished by any of the existing aromatic mechanisms.
The dominant particle phase species predicted by the mechanism are glyoxal oligomers (organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and, for the low concentration experiments, unsaturated hydroxyl nitro-acids. The relative amounts of these products vary depending on initial experimental conditions. In general, with decreasing toluene/NO ratios, the relative amount of total organic nitrates and acylperoxy nitrates in the particle phase increases, the mass fraction of total oligomers and organic peroxides decreases. It is also important to note that the relative amount of different SOA species dramatically changes with time. The model also well predicts the SOA mass concentrations observed from the European Photoreactor (EUPHORE) and smog chambers at the California Institute of Technology (Caltech). But to implement the developed mechanism into the regional airshed model, it would be desirable to reduce the reaction steps and number of represented species.
It is recommended that future studies should focus on: 1. reaction mechanisms that contribute the rapid particle formation in toluene/NOx system, 2. identification of particle phase oligomers, 3. measurement of organic nitrates and acylperoxy nitrates, 4. chamber experiments with reactant concentrations at ambient levels, and 5. combining aromatics and monoterpenes mechanisms into one unified mechanism.
Work over this past year: Since the fall of 2006 we have conducted a number of experiments with toluene and o-xylene and an eleven component mixture of alkanes and alkenes (Called the UNC Mix). The purpose was to see if we could provide a realistic data set to model SOA formation from toluene in a realistic gas phase hydrocarbon environment. One of the first modeling observations was that SOA formation from the mechanism developed for this project was that toluene + oxides of nitrogen (NOx,) in sunlight was higher, than if the same amount of toluene and NOx was reacted in an environment of a volatile atmospheric hydrocarbon mixture.
To test this observation, experiments were conducted in the Fall of 2006 in the 270m3 UNC dual outdoor smog chamber. Experiments had concentrations of 0.5 to 1 ppmC of toluene, 2 to 3 ppmC of UNC Mix and 0.05 to 0.2 ppm of NOx
Key observations were:
- When just UNC mix + NOx was reacted under clear sunlight in the chamber, no or very little SOA was observed.
- In experiments that had initial background chamber aerosol concentrations in the 3 μg/m3 range and initial chamber dew points of 15°C, a dramatic increase in the amount of generated SOA (~6 μg/m3) was observed over the Fall 2006 dry chamber experiments.
- When the chamber air was not very dry, but initial seed aerosol concentrations were low, SOA formation was reduced.
- The presence of background diesel exhaust particles (~15μg/m3) enhanced SOA formation, but the relative dryness of the air did not impact SOA formation.
- When ammonium sulfate was substituted for diesel soot particles in similar systems, under dry conditions, very little SOA was generated. Under more humid initial conditions (dew point 13°C), however, considerable SOA was generated. It is expected that dry ammonium sulfate particles are not conducive to gas-particle petitioning, while a under humid conditions, a water layer exists on the particles which is receptive to the partitioning of highly oxygenated gas phase toluene reactions products.
Future Activities:
We will continue to explore the impact of different aerosol seed backgrounds on the formation or aromatic SOA.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 8 publications | 6 publications in selected types | All 6 journal articles |
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Type | Citation | ||
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Hu D, Tolocka M, Li Q, Kamens RM. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NOx and natural sunlight. Atmospheric Environment 2007;41(31):6478-6496. |
R831084 (2006) R831084 (2007) R831084 (Final) |
Exit Exit Exit |
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Lee S, Kamens RM. Particle nucleation from the reaction of α-pinene and O3. Atmospheric Environment 2005;39(36):6822-6832. |
R831084 (2005) R831084 (2006) R831084 (2007) R831084 (Final) |
Exit Exit Exit |
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Leungsakul S, Jaoui M, Kamens RM. Kinetic mechanism for predicting secondary organic aerosol formation from the reaction of d-limonene with ozone. Environmental Science & Technology 2005;39(24):9583-9594. |
R831084 (2007) R828176 (Final) |
Exit Exit Exit |
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Leungsakul S, Jeffries HE, Kamens RM. A kinetic mechanism for predicting secondary aerosol formation from the reactions of d-limonene in the presence of oxides of nitrogen and natural sunlight. Atmospheric Environment 2005;39(37):7063-7082. |
R831084 (2005) R831084 (2006) R831084 (2007) R831084 (Final) R828176 (Final) |
Exit Exit Exit |
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Li Q, Hu D, Leungsakul S, Kamens RM. Large outdoor chamber experiments and computer simulations: (I) secondary organic aerosol formation from the oxidation of a mixture of d-limonene and α-pinene. Atmospheric Environment 2007;41(40):9341-9352. |
R831084 (2007) R831084 (Final) |
Exit Exit Exit |
Supplemental Keywords:
Secondary organic aerosol formation, aromatics, modeling, organic particle formation,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, Atmospheric Sciences, environmental measurement, Toluene, gas phase chemistry, particle phase reactions, organic chemistry, secondary organic aerosol, aromatic compounds, aerosol analyzersProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.